Inflammation is the response of vascularized tissues to infection or injury. Clinically it is accompanied by four classic signs: redness, heat, pain and swelling. Its course may be acute or chronic.
At the cellular level, inflammation involves the adhesion of leukocytes (white blood cells) to the endothelial wall of blood vessels and their infiltration into the surrounding tissues. (Harlan, 1985.) Acute inflammation is characterized by the adhesion and infiltration of polymorphonuclear leukocytes (PMNs). (Harlan, 1987 and Malech and Gallin, 1987.) PMN accumulation in the tissues reaches its peak between two and one half to four hours after an inflammatory stimulus and ceases by about twenty-eight hours (Bevilacqua and Gimbrone, 1987). In contrast, chronic inflammation is characterized by the adhesion and infiltration of other leukocytes, especially monocytes and lymphocytes.
In normal inflammation, the infiltrating leukocytes phagocytize invading organisms or dead cells, and play a role in tissue repair and the immune response. However, in pathologic inflammation, infiltrating leukocytes can cause serious and sometimes deadly damage. Rheumatoid arthritis and atherosclerosis are examples of chronic inflammatory diseases in which mononuclear leukocytes infiltrate the tissues and cause damage. (Hough and Sokoloff, 1985 and Ross, 1986.) Multiple organ failure syndrome, adult respiratory distress syndrome (ARDS), and ischemic reperfusion injury are acute inflammations in which infiltrating PMNs cause the damage (Harlan, 1987 and Malech and Gallin, 1987). In multiple organ failure syndrome, which can occur after shock such as that associated with severe burns, PMN-mediated damage exacerbates the injury. In ARDS, PMNs cause the lungs to fill with fluid, and the victim may drown. In ischemic reperfusion injury, which occurs when tissue cut off from the supply of blood is suddenly perfused with blood (for example after heart attack, stroke, or limb re-attachment), PMN adhesion causes serious tissue damage (Harlan, 1987).
Recognizing that leukocyte infiltration is the cause of much inflammation-related pathology and that leukocyte adhesion is the first step in infiltration, investigators have recently focused attention on the mechanism of leukocyte binding to the endothelial cell surface. Studies show that binding is mediated by cell-surface molecules on both endothelial cells and leukocytes which act as receptor and ligand (Harlan et al., 1987; Dana et al., 1986; and Bevilacqua et al., 1987a).
During the course of inflammation, certain inflammatory agents can act on the leukocytes, making them hyperadhesive for endothelium. Known inflammatory agents include leukotriene-B4 (LTB4), complement factor 5a (C5a), and formyl-methionyl-leucyl-phenylalanine (FMLP). These agents activate a group of proteins called LeuCAMs. The LeuCAMs are dimers of the CD11 and CD18 proteins. One of the LeuCAMs, CD11a/CD18 (also called LFA-1) binds to a receptor on endothelial cells called ICAM1 (intercellular adhesion molecule 1). (Harlan, 1985 and Dana et al., 1986.) Investigators have shown that monoclonal antibodies (Moabs) to LeuCAMs inhibit PMN adhesion to endothelium both in vitro and in vivo. (Arfors, 1987; Vedder et al., 1988; and Todd, 1989.)
Other inflammatory agents act directly on endothelial cells to substantially augment leukocyte adhesion. These agents include the cytokines interleukin-1 (IL-1), lymphotoxin (LT) and tumor necrosis factor (TNF), as well as the bacterial endotoxin, lipopolysaccharide (LPS). For example, IL-1 has been shown to stimulate adhesion of PMNs, monocytes, and the related cell lines HL-60 (PMN-like) and U937 (monocyte-like), to human endothelial cell monolayers. The action is both time-dependent and protein-synthesis dependent. (Bevilaqcua et al., 1987a; Bevilacqua et al., 1987b; and Bevilacqua et al., 1985.)
Current evidence indicates that these agents induce a group of molecules on the endothelial cell surface called ELAMs (endothelial cell-leukocyte adhesion molecules). To date investigators have identified two of these molecules, intercellular adhesion molecule 1 (ICAM1) and endothelial cell-leukocyte adhesion molecule 1 (ELAM1). (Simmons et al., 1988; Staunton et al., 1988; and Bevilaqcua et al., 1987b.) ICAM1 is found on many cell types, and its expression on vascular endothelium is strongly upregulated both in vitro and in vivo by the inflammatory cytokines interleukin 1 (IL-1), tumor necrosis factor-.alpha. (TNF), and gamma interferon (IFN-.gamma.). (Pober et al., 1986; Dustin and Springer, 1988; and Cotran and Pober, 1988.)
ELAM1 was initially detected and characterized by a monoclonal antibody (MoAb) that partially blocked PMN adhesion to cytokine-treated human umbilical vein endothelial cells (HUVECs). ELAM1 is a 116 kD cell surface glycoprotein rapidly synthesized by HUVECs in response to the inflammatory cytokines IL-1 or TNF, but not IFN-.gamma.. (Bevilacqua et al., 1987b.) Unlike ICAM1, ELAM1 appears to be expressed only in endothelium, and its expression is transient even in the continued presence of cytokine. Like ICAM1, ELAM1 is present at inflammatory sites in vivo. Immunohistologic studies show that it exists at sites of acute, but not chronic, inflammation, and is absent from the non-inflamed vessel wall. (Cotran et al., 1986 and Cotran and Pober, 1988.) Therefore, ELAM1 appears to be a major mediator of PMN adhesion to the inflamed vascular wall in vivo. Importantly, the presence of ELAM1 on the cell surface follows the natural course of acute inflammation, appearing a few hours after stimulation and gradually dissipating within a day. (Bevilacqua et al., 1987b.)
Indirect evidence suggests that other ELAMs exist. Although inflammatory agents induce binding of PMNs, monocytes, and lymphocytes to endothelium in vitro, Moabs against ELAM1 inhibit only the binding of PMNs and related cells. (Bevilacqua and Gimbrone, 1987.) Furthermore, maximal accumulation of lymphocytes and monocytes at sites of inflammation in vivo occurs at about twenty-four hours, when ELAM1 expression has returned to basal levels. On the basis of such information investigators inferred the presence of other ELAMs that mediate binding of these lymphocytes and monocytes. (Bevilacqua et al., 1987b.) As set forth in detail below, we have characterized and cloned two more ELAMs, designated VCAM1 and VCAM1b, that mediate binding of lymphocytes to endothelial cells. ELAMs accordingly may be regarded as a family of molecules.
A growing body of evidence indicates that ELAMs may play important roles in a wide range of pathological states involving cell-cell recognition, including tumor invasion, metastasis and viral infection. (Harlan, 1985; Wallis and Harlan, 1986; Bevilacqua et al., 1987a; and Cotran and Pober, 1988.)
The adhesion of leukocytes to cells expressing ELAMs suggests the existence on leukocytes of ELAM ligands. One such molecule is the ICAM1 ligand, lymphocyte function associated antigen 1 (LFA1). LFA1 is one of a trio of heterodimeric molecules known as the .beta.2 integrins or the CD11/18 family. (Dustin et al., 1986; Rothlein et al., 1986; and Marlin and Springer, 1987.) Recent studies show that the ICAM1/LFA1 pathway plays a role in both lymphocyte and polymorphonuclear leukocyte (PMN) adhesion to endothelial cells in vitro. (Dustin and Springer, 1988; Smith et al., 1989.) We report here the isolation of a molecule involved in leukocyte adhesion to endothelial cells (MILA) which may prove to be an ELAM1 ligand. The molecule, designated CDX, is a 175 kD protein and was isolated from HL-60 cells. Monoclonal antibodies that recognize CDX inhibit the binding of PMNs and HL-60 cells to ELAM1-expressing cells. Furthermore, CDX is present on leukocyte cell types known to adhere to ELAM1 and is absent from leukocyte cell types and other cell types that do not adhere to ELAM1. Thus, CDX is a molecule expressed on certain leukocytes that plays an important role in ELAM1-mediated leukocyte-endothelial cell adhesion.
We also report the identification of a VCAM1 and VCAM1b ligand, VLA4. (Hemler and Takada, EP 330,506). Antibodies specific for the .alpha..sup.4 and .beta..sub.1 subunits of VLA4 completely eliminate binding of VLA4-expressing cells to VCAM1.
Because leukocyte adhesion to the vascular wall is the first step in inflammation, therapies directed to preventing this step are attractive for the treatment of pathologic inflammation. Clinicians are already testing, with some success, therapies based on inhibiting leukocyte-mediated adhesion. One such approach involves Moab binding to the leukocyte cell-surface complex, CD11/CD18, to inhibit PMN adhesion. (Arfors et al., 1987; Vedder et al., 1988; and Todd et al., 1989.)
We believe that alternative therapies for preventing leukocyte adhesion, based on endothelial cell-mediated binding, and on ELAMs and MILAs (including ELAM ligands), in particular, are more promising. The ELAM system is particularly appealing for two reasons: First, because ELAM expression on endothelial cells is induced rather than constitutive, ELAMs are concentrated at sites of inflammation and are limited in number. This means that adhesion inhibitors need act only locally and, consequently, would be effective at lower doses than inhibitors directed to constitutively expressed molecules. Second, ELAM binding is selective for different leukocyte classes. For example, ELAM1 binds PMNs, and VCAM1 binds lymphocytes. Therefore, these therapies would be specific for certain classes of leukocytes and would not affect the circulation or migration of other leukocyte classes. Furthermore, for the above reasons, such therapies may prove to be cheaper and less toxic.
ELAM-based approaches to therapy require, as starting materials, both ELAMs and MILAs in highly purified form, free of normally associated animal proteins. There is also a need for methods to produce these molecules. These and other needs have now been met as described herein, by isolating DNA sequences that code on expression for particular adhesion molecules and by constructing recombinant DNA molecules and expression vehicles for their production.